Plant Quiz

Well, I could say that this is a plant quiz, but I really am just confused. I would appreciate any help. This is from a mitigation wetland in Central Indiana (Madison County), but I don't think it was seeded. I have been watching it all winter and waiting for it to bloom. It only grows in standing water, although I don't think it fits the definition of true aquatic, since it doesn't have any thin leaves and does emerge from the water. In the winter, it would be in 6" deep water and have basal leaves that float to the surface. You can see some of the basal leaves in one of the pictures. Now it is blooming. The petals are tiny (<6mm) I have assumed it is a buttercup, but I can't get anything to fit.